Devices such as flat-panel displays typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods, which involve high substrate temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a flexible substrate and carbon nanotube or organic electroconductive polymer layers as an electrode. Such plastic electronics allow low cost devices with new properties. Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous hopper or roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the “roll to roll” fabrication of electronic devices which are more flexible, lower cost, and lower weight. Touchscreens (also referred to as touch panels or touch switches) are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers. FIG. 1 shows a typical prior art resistive-type touchscreen 10 comprising a first electrode 15 that is on the side of the touchscreen that is nearer to the device that is referred herein below as the device side electrode and a second electrode 16 that is on the side of the touchscreen that is nearer to the user that is referred herein below as the touch side electrode. Device side electrode 15 comprises a transparent substrate 12, having a first conductive layer 14. Touch side electrode 16 comprises a transparent support 17, that is typically a flexible transparent support, and a second conductive layer 18 that is physically separated from the first conductive layer 14 by dielectric (insulating) spacer elements 20. A voltage is developed across the conductive layers. The conductive layers 14 and 18 have a resistance selected to optimize power usage and position sensing accuracy. Deformation of the touch side electrode 16 by an external object such as a finger or stylus causes the second conductive layer 18 to make electrical contact with first conductive layer 14, thereby transferring a voltage between the conductive layers. The magnitude of this voltage is measured through connectors (not shown) connected to metal bus bar conductive patterns (not shown) formed on the edges of conductive layers 18 and 14 to locate the position of the deforming object.
ITO is commonly employed as the transparent conductive layer on the device side and touch side electrodes. However, ITO tends to crack under stress and with the result that the conductivity of the electrodes, especially for the touch side electrode, is diminished and the performance of the touchscreen degraded. More flexible conductive polymer-containing layers have also been considered for this application, but these conductive polymers are softer and less physically durable than ITO and therefore such conductive layers tend to degrade after repeated contacts.
Single wall carbon nanotubes (SWCNTs) are essentially graphene sheets rolled into hollow cylinders thereby resulting in tubules composed of sp2 hybridized carbon arranged in hexagons and pentagons, which have outer diameters between 0.4 nm and 10 nm. These SWCNTs are typically capped on each end with a hemispherical fullerene (buckyball) appropriately sized for the diameter of the SWCNT. However, these end caps may be removed via appropriate processing techniques leaving uncapped tubules. SWCNTs can exist as single tubules or in aggregated form typically referred to as ropes or bundles. These ropes or bundles may contain several or a few hundred SWCNTs aggregated through Van der Waals interactions forming triangular lattices where the tube-tube separation is approximately 3-4 Å. Ropes of SWCNTs may be composed of associated bundles of SWCNTs.
The inherent properties of SWCNTs make them attractive for use in many applications. SWCNTs can possess high (e.g. metallic conductivities) electronic conductivities, high thermal conductivities, high modulus and tensile strength, high aspect ratio and other unique properties. Further, SWCNTs may be metallic, semi-metallic, or semiconducting dependant on the geometrical arrangement of the carbon atoms and the physical dimensions of the SWCNT. To specify the size and conformation of single-wall carbon nanotubes, a system has been developed, described below, and is currently utilized. SWCNTs are described by an index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped into the form of a cylinder. When n=m e.g. (n, n), the resultant tube is said to be of the “arm-chair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. When m=0, the resultant tube is said to be of the “zig zag” or (n,0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig zag pattern. Where n≠m and m≠0, the resulting tube has chirality. The electronic properties are dependent on the conformation; for example, armchair tubes are metallic and have extremely high electrical conductivity. Other tube types are semimetals or semi-conductors, depending on their conformation. SWCNTs have extremely high thermal conductivity and tensile strength irrespective of the chirality. The work functions of the metallic (approximately 4.7 eV) and semiconducting (approximately 5.1 eV) types of SWCNTs are different.
Similar to other forms of carbon allotropes (e.g. graphite, diamond) these SWCNTs are intractable and essentially insoluble in most solvents (organic and aqueous alike). Thus, SWCNTs have been extremely difficult to process for various uses. Several methods to make SWCNTs soluble in various solvents have been employed. One approach is to covalently functionalize the ends of the SWCNTs with either hydrophilic or hydrophobic moieties. A second approach is to add high levels of surfactant and/or dispersants (small molecule or polymeric) to help solubilize the SWCNTs.
Lavin et al. in U.S. Pat. No. 6,426,134 disclose a method to form polymer composites using SWCNTs. This method provides a means to melt extrude a SWCNT/polymer composite wherein at least one end of the SWCNT is chemically bonded to the polymer, where the polymer is selected from a linear or branched polyamide, polyester, polyimide, or polyurethane. This method is limited to melt extrusion which can limit opportunities for patterning or device making. The chemically bonded polymers identified typically have high molecular weights and could interfere with some material properties of the SWCNTs (e.g. electronic or thermal transport) via wrapping around the SWCNTs and preventing tube-tube contacts.
Connell et al in US Patent Application Publication 2003/0158323 A1 describes a method to produce polymer/SWCNT composites that are electronically conductive and transparent. The polymers (polyimides, copolyimides, polyamide acid, polyaryleneether, polymethylmethacrylate) and the SWCNTs or MWCNTs are mixed in organic solvents (DMF, N,N-dimethlacetamide, N-methyl-2-pyrrolidinone, toluene,) to cast films that have conductivities in the range of 10−5-10−12 S/cm with varying transmissions in the visible spectrum. Additionally, monomers of the resultant polymers may be mixed with SWCNTs in appropriate solvents and polymerized in the presence of these SWCNTs to result in composites with varying weight ratios. The conductivities achieved in these polymer composites are several orders of magnitude too low for use in most electronic devices as electronic conductors or EMI shields. Additionally, the organic solvents used are toxic, costly and pose problems in processing. Moreover, the polymers used or polymerized are not conductive and can impede tube-tube contact further increasing the resistivity of the composite.
Kuper et al in Publication WO 03/060941A2 disclose compositions to make suspended carbon nanotubes. The compositions are composed of liquids and SWCNTs or MWCNTs with suitable surfactants (cetyl trimethylammonium bromide/chloride/iodide). The ratio by weight of surfactant to SWCNTs given in the examples range from 1.4-5.2. This method is problematic, as it needs extremely large levels of surfactant to solubilize the SWCNTs. The surfactant is insulating and impedes conductivity of a film deposited from this composition. The surfactant may be washed from the film but this step adds complexity and may decrease efficiency in processing.
Papadaopoulos et al. in U.S. Pat. No. 5,576,162 describe an imaging element, which comprises carbon nanofibers to be used primarily as an anti-static material within the imaging element. These materials do not provide a highly transparent and highly conductive (low sheet resistance, RS) layer.
Smalley et al in U.S. Pat. No. 6,645,455 disclose methods to chemically derivatize SWCNTs to facilitate solvation in various solvents. Primarily the various derivative groups (alkyl chains, acyl, thiols, aminos, aryls etc.) are added to the ends of the SWCNTs. The side-walls of the SWCNTs are functionalized primarily with fluorine groups resulting in fluorinated SWCNTs. The solubility limit of such “fluorotubes” in 2-propanol is approximately 0.1 mg/mL and in water or water/acetone mixtures the solubility is essentially zero. The fluorinated SWCNTs were subjected to further chemical reactions to yield methylated SWCNTs and these tubes have a low solubility in Chloroform but not other solvents. In addition, the method discloses functionalization of the tubule ends with various functionalization groups (acyl, aryl, aralkyl, halogen, alkyl, amino, halogen, thiol. Further, the sidewall functionalization is done with fluorine only, which gives limited solubility in alcohols. Additionally, the fluorinated SWCNTs are insulators due to the fluorination. Moreover, the chemical transformations needed to add these functional groups to the end points of the SWCNTs require additional processing steps and chemicals which can be hazardous and costly.
Smalley et al. in U.S. Pat. No. 6,683,783 disclose methods to purify SWCNT materials resulting in SWCNTs with lengths from 5-500 nm. Within this patent, formulations are disclosed that use 0.5 wt % of a surfactant, Triton X-100 to disperse 0.1 mg/mL of SWCNT in water. The method discloses functionalization of the tubule ends with various functionalization groups (acyl, aryl, aralkyl, halogen, alkyl, amino, halogen, thiol) but the end functionalization alone may not be enough to produce viable dispersions via solubilization. The chemical transformations needed to add these functional groups to the end points of the SWCNTs require additional processing steps and chemicals which can be hazardous and costly.
Rinzler et al. in PCT Publication WO2004/009884 A1 disclose a method of forming SWCNT films on a porous membrane such that it achieves 200 ohms/square and at least 30% transmission at a wavelength of 3 um. This method needs a porous membrane (e.g. polycarbonate or mixed cellulose ester) with a high volume of porosity with a plurality of sub-micron pores as a substrate. Further, the membrane is set within a vacuum filtration system. Moreover, the weight percent of the dispersion used to make the SWCNT film was 0.005 mg/mL in an aqueous solution.
Blanchet-Fincher et al in Publication WO 02/080195A1 and in US 20040065970 A1 illustrate high conductivity compositions composed of polyaniline (PANI) and SWCNTs or MWCNTs and methods to deposit such compositions from a donor element onto a receiver substrate. The nitrogen base salt derivative of emeraldine polyaniline is mixed with SWCNTs in organic solvents (toluene, xylene, turpinol, aromatics) and cast into films with conductivity values of 62 S/cm (1 wt % SWCNT in PANI) and 44 S/cm (2 wt % SWCNT in PANI). These films alternatively may be produced as part of a multi-layer donor structure suitable as use for a material transfer system. The PANI/SWCNT composite are transferred from the donor sheet to a suitable receiver substrate in imagewise form. PANI is a highly colored conductive polymer and may result in a conductive composite with unsatisfactory displays.
Hsu in WO 2004/029176 A1 disclose compositions for electronically conducting organic polymer/nanoparticle composites. Polyaniline (Ormecon) or PEDT (Baytron P) are mixed with Molybdenum nanowires or carbon nanotubes (8 nm diameter, 20 um length, 60 S/cm). The compositions disclosed in this invention have marginal conductivity.
Arthur et al in PCT Publication WO 03/099709 A2 disclose methods for patterning carbon nanotubes coatings. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water (which may include viscosity modifying agents) are spray coated onto substrates. After application of the SWCNT coating, a binder is printed in imagewise fashion and cured. Alternatively, a photo-definable binder may be used to create the image using standard photolithographic processes. Materials not held to the substrate with binder are removed by washing. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water with viscosity modifying agents are gravure coated onto substrates. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water are spray coated onto substrates. The coated films are then exposed through a mask to a high intensity light source in order to significantly alter the electronic properties of the SWCNTs. A binder coating follows this step. The dispersion concentrations used in these methods make it very difficult to produce images via direct deposition (inkjet etc.) techniques. Further, such high solvent loads due to the low solids dispersions create long process times and difficulties handling the excess solvent. In addition, these patterning methods are subtractive processes, which unnecessarily waste the SWCNT material via additional removal steps thereby incurring cost and process time.
Luo et al in International Publication Number WO2005/086982A2 and US Patent Publication Number US2005/0209392A1 disclose use of carbon nanotubes in general for LC displays, touchscreens, and EMI shielding windows.
Transparent electronically-conductive layers (TCL) of metal oxides such as indium tin oxide (ITO), antimony doped tin oxide, and cadmium stannate (cadmium tin oxide) have been used in the manufacture of electrooptical display devices such as liquid crystal display devices (LCDs), electroluminescent display devices, photocells, touchscreens, solid-state image sensors and electrochromic windows or as components of these devices such as electromagnetic interference (EMI) shielding.
As indicated herein above, the art discloses a wide variety of electronically conductive TCL compositions that can be incorporated in electronic devices. However, the stringent requirements of high transparency, low sheet resistance, flexibility, and robustness under repeated contacts demanded by modern display devices and, especially, touchscreens is extremely difficult to attain with the TCL compositions described in the prior art. Thus, there is still a critical need for transparent conductors that can be coated roll-to-roll on a wide variety of substrates under typical manufacturing conditions using environmentally desirable components. In addition to providing superior touchscreen electrode performance, the TCL layers also must be highly transparent, must resist the effects of humidity change, be physically robust, and be manufacturable at a reasonable cost.